g77 comes from the compatibility package compat-gcc-34-g77. You should install the corresponding gcc compatibility package which is called compat-gcc-34 and modify Makefile.gnu to use gcc34 instead of gcc. However, this combination is quite likely to produce a CHARMM executable which would not be fully functional, f.e. the large memory allocations might fail.
The other possibility is to modify the same Makefile.gnu and change g77 into gfortran, while leaving gcc as is. This is more likely to produce a well behaving executable than g77+gcc34.